I am just trying to design a widget which is supposed to use a TabControl.In CEED, I can add a TabControl based widget.

But as soon as I am adding something (let's say a DefaultWindow with a Vanilla/Button in it) to that TabControl in CEED, it does not display (though it does seem to try to add a panel button). When I then try to click on the added DefaultWindow, I get an error message:"can't find widget manipulator of path 'DefaultWindow'"

If that layout is stored and reloaded, I get:"layout loading has been aborted since auto window '__auto_btnDefaultWindow' could not be referenced."

So, am I doing something wrong here or can TabControls not be filled via CEED?

Yup, TabControl is not supported yet. I recently implemented LayoutContainers, that works if you use CEED from the repository. TabControls are a bit special and manipulators need to be changed to support them.

Unfortunately I am too busy to tackle it right now. I may have time to help somebody who'd like to fix this.